Method of joining two objects

ABSTRACT

A method for joining two objects by anchoring an insert portion provided on one of the objects in an opening provided on the other one of the objects. The anchorage is achieved by liquefaction of a thermoplastic material and interpenetration of the liquefied material and a penetrable material, the two materials being arranged on opposite surfaces of the insert portion and the wall of the opening. Before such liquefaction and interpenetration, an interference fit is established in which such opposite surfaces are pressed against each other, and, for the anchoring, mechanical vibration energy and possibly a shearing force are applied, wherein the shearing force puts a shear stress on the interference fit.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention is in the field of mechanical engineering and constructionand concerns a method for joining two objects. One of the objectsincludes an insert portion and the other object includes an opening,and, for the two objects to be joined, the insert portion is anchored inthe opening, wherein, in the anchoring area, one of the objects includesa solid material having thermoplastic properties, and the other objectincludes a solid material, which is penetrable by the material havingthermoplastic properties when liquefied. The penetrable material is,e.g., fibrous or porous, it is, e.g., a structural foam.

Description of Related Art

From e.g. the publications WO 96/01377 (Createc), WO 98/042988(Woodwelding) and WO 2006/002569 (Woodwelding) or WO 2008/080238 it isknown to anchor inserts including materials having thermoplasticproperties in fibrous or porous materials such as e.g. chipboard orwood. For such anchoring, the insert is positioned relative to theopening, and then mechanical vibration, in particular ultrasonicvibration, and a force directed for pressing the insert into the openingare simultaneously applied to the insert. In the step of positioning theinsert, no relevant force is used, i.e., on application of the vibratoryenergy, the positioned insert will vibrate freely, or will, due to thenamed force pressing it against the fibrous or porous material, transmitthe vibratory energy to the latter. In the step of applying vibrationand force, the material having thermoplastic properties is liquefied dueto friction heat at least where in contact with the fibrous or porousmaterial and it penetrates into the fibrous or porous material of thewalls of the opening and forms on re-solidification a positive fitconnection with the porous or fibrous material.

According to the publication WO 96/01377, the insert and the blindopening in which the insert is to be anchored are adapted to each othersuch that the insert, on being positioned relative to the opening, doesnot reach the bottom of the opening. This is achieved by designing theinsert and/or the opening to be tapering either continuously orstep-wise. Simultaneously with the application of the ultrasonicvibration the insert is further advanced into the opening through theaction of a pressing force, which results in liquefaction of thematerial having thermoplastic properties mainly on lateral insert sides.In a similar method a slightly oversized insert is positioned on themouth of the opening and, under the action of the pressing force and thevibration, is advanced into the opening and laterally anchored therein.In both cases it is necessary to move the bulk of the insert in thedirection of the depth of the opening during the application of thevibration.

According to the publications WO 98/042988 and WO 2006/002569, theinsert and the blind opening in which the insert is to be anchored areadapted to each other such that the insert on being positioned in theopening sits on the bottom of the opening. During the application of theultrasonic vibration, the insert is pressed against this bottom. Thisresults in liquefaction of the material having thermoplastic propertiesin particular in the region of the distal insert end, i.e. in asubstantially distal anchorage at the bottom of the opening but hardlyany lateral anchoring, wherein the bulk of the insert is not or onlyvery little moved during the application of the vibration.

U.S. Pat. No. 5,879,115 teaches a method and an apparatus for joining aplastic insert to a plastic base having a recess therein. The processrelies on plastic material of both, the insert and the base to melt andto flow into spaces where the plastics of the insert and the base aremelded together into a puddle to yield, after re-solidification, anintegral joint (a weld). Applications of this approach are restricted tobonds between two objects that are both of thermoplastic material, thethermoplastic materials being capable of intermixing integrally witheach other.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an alternative methodfor joining two objects, wherein one of the objects includes an insertportion and the other one includes an opening, wherein, for the joining,the insert portion is anchored in the opening, and wherein, at least inareas in which such anchoring is desired, one of the objects includes asolid material having thermoplastic properties, and the other objectincludes a solid material, which is penetrable by the material havingthermoplastic properties when liquefied.

A further object of the invention is to improve the above brieflydescribed, known methods in which such joining is achieved with the aidof energy provided in the form of mechanical vibration, in particularultrasonic vibration. Using the method according to the invention it isto be possible to achieve with the simplest design of insert and openingand within a minimum time of vibration application a particularlylateral anchorage of a very high uniformity even if the desiredanchoring area is relatively large.

Generally speaking, the above named objects are achieved byestablishing, in a first method step, an interference fit between theinsert portion and the wall, in particular the lateral wall, of theopening in which the insert portion is to be anchored, and only thenapplying the energy which causes the liquefaction, which is suchachieved mainly where the interference fit is active. The anchorageachieved with this method is of a similar strength (per anchorage areaunit) as the anchorage achieved with the above briefly discussed methodaccording to WO 96/01377, but it can be carried out without thenecessity of moving the bulk of the insert portion over a relevantdistance within the opening during the anchoring step, and thisindependent on the location in which anchorage is desired. This factrelevantly reduces the time necessary for achieving the anchorage.Furthermore, as liquefaction starts simultaneously in all locations inwhich anchoring is desired, the achieved anchorage is very homogeneous.

According to the invention, a first object including a first materialand a second object including a second material are provided, whereinthe first material is solid (at ambient temperature) and includesthermoplastic properties (i.e., it is liquefiable with the aid ofthermal energy; in the following this material is called “thermoplasticmaterial”), and the second material is also solid (at ambienttemperature) and it is penetrable by the first material when the latteris in a liquefied state (i.e. the second material is fibrous or porous,it includes penetrable surface structures or it cannot resist suchpenetration under pressure). Especially, the second material is suchthat it does not become flowable under the conditions that apply whenthe first material penetrates the surface structures. For example, thesecond material may be of a material that does not have thermoplasticproperties, i.e. a material different from a thermoplastic material. Thesecond material may be such that it does not undergo a reversibleliquefaction process, which means that it is of a material incapable ofundergoing a reversible liquefaction process or of a material that has amelting temperature substantially above a temperature at which the firstmaterial becomes flowable. For example, if the second material ismeltable, its melting temperature or glass transition temperature may behigher than a glass transition temperature or melting temperature of thefirst material by at least 50° C. or at least 80° C. or at least 100° C.

According examples of second materials are wood-based materials such aschipboard (“chipboard” in this text includes any composite materialsmanufactured by mixing wood particles of any shape with adhesives,independent of the product's shape, including for example orientedstrand board), High Density Fiber board (HDF), Medium Density Fiberboard (MDF), or wood, or metallic or ceramic foams or possibly openporous structures of a material based on a not thermoplastic(thermosetting) polymer.

Either one of the first and second object includes an opening and theother one includes an insert portion to be anchored in the opening,wherein the named materials constitute at least part of the surfaces ofthe insert portion and of the wall of the opening. The opening and theinsert portion are dimensionally adapted to each other for aninterference fit (press-fit), i.e. the insert portion is oversized atleast locally compared with the opening, resulting in at least localpressure between insert portion and opening wall when the insert has adesired position within the opening, i.e. resulting in elasticcompression of the insert portion and/or the wall of the opening.Therein the named first and second materials are arranged opposing eachother at least in parts of areas of such compression.

Especially, the second object may include the opening and the firstobject may include the insert portion.

In a method step of establishing the interference fit, the insertportion is positioned in the desired position within the opening whereit is retained by the above named interference fit. For achieving theinterference fit, i.e. for generating the compression between insertportion and opening wall an interference force is necessary, either forforcing the insert portion into the undersized opening or for pressingwall sections of the opening against the insert portion. The magnitudeof the interference force correlates substantially with the strength andarea of the interference fit and it is mainly dependent on and limitedby the relative dimensions of insert portion and opening and on thecompressibility of either one or both of the two materials.

In an anchoring step being carried out after the step of establishingthe interference fit, energy is applied to one or the other of theobjects, wherein the energy is to act as heat in particular in areas(anchoring areas) in which, due to the interference fit, surface areasof insert portion and opening wall are pressed against each other andinclude one each of the thermoplastic and the penetrable material. Theheat causes the thermoplastic material to liquefy and the pressure ofthe interference fit causes interpenetration of the two materials,wherein the interference fit is at least partly relaxed.

If the energy is applied as mechanical vibration (e.g. ultrasonicvibration), the interference fit is to be overcome, such that, at leastin the named areas, the vibration causes friction and therewith frictionheat between the insert portion and the wall of the opening. Forovercoming the interference fit, it is necessary to apply a shearingload between the insert portion and the opening wall, wherein thisshearing load may be caused by strong enough vibration of the one ofinsert portion or opening wall relative to the other one, or by thevibration and an additional shearing force acting between the twoobjects. For preventing undesired movement, in particular translationalmovement of the two objects relative to each other due to the shearingforce, it may be necessary to counteract the latter in a suitablemanner.

In a further method step the supply of energy is stopped until thethermoplastic material liquefied and dislocated in the anchoring step isre-solidified, whereby in the interpenetration area a sort of compositematerial is formed, which connects the two objects in a positive fitconnection.

The energy necessary for liquefying the thermoplastic material in theanchoring step is supplied, as above discussed, to either one of the twoobjects preferably in the form of mechanical vibration, in particularultrasonic vibration, to be transformed into friction heat at theinterface between the insert portion and the wall of the opening. Thevibration preferably has a main vibration direction parallel to thesurfaces of insert portion and opening wall where the interference fitis active. Preferred therefore are, for achieving lateral anchorage,longitudinal vibrations substantially parallel to the depth of theopening or rotary vibrations with an axis substantially parallel to thedepth of the opening. The above named additional shearing force, ifapplied, is preferably directed parallel to the main vibrationdirection, i.e. for the above named two cases it acts parallel to thedepth of the opening or as torque with an axis parallel to the depth ofthe opening.

Other sorts of energy such as, e.g., irradiation with electromagneticenergy for which suitable absorbing means are to be provided at thelocations where the interference fit is active, or corresponding heating(e.g. inductive or resistive heating) are applicable also.

As mentioned above, the thermoplastic material and the penetrablematerial taking part in the anchorage may be present only on selectedsurfaces of the insert portion and on walls of the opening. However theymay also constitute larger portions of the two objects which may includefurther portions of different materials or may fully consist of eitherthe thermoplastic material or the penetrable material.

In areas in which the interference fit acts either one of the twosurfaces being pressed together may include structures functioning asenergy directors, i.e. point-shaped or line-shaped elements protrudingfrom a principal surface. These energy directing structures may or maynot disrupt the opposite surface when the interference fit isestablished and need to be correspondingly taken into account whencalculating the oversize of the insertion portion.

The penetrable material and the thermoplastic material need to havemechanical properties (under the conditions of the step for establishingthe interference fit, i.e. usually at ambient temperatures) to becapable to stand the pressure of the interference fit. Creeping ofeither one of the two materials, which impairs the interference fit, ispreferably to be prevented. This is achieved by selecting materials withsubstantially no creeping tendency at the temperature of the step ofestablishing the interference fit (amorphous polymers below their glasstransition temperature or preferably at least 50° C. below their glasstransition temperature and part crystalline polymers below their meltingtemperature) or by keeping a time interval between the step ofestablishing the interference fit and the step of anchoring short enoughfor starting the anchoring step before the interference fit is too muchimpaired by the creeping (the shorter the named time interval is, themore creeping tendency of either one of the materials can be tolerated,i.e. for polymers: the nearer the used temperature can be to the abovegiven temperature limits). If the energy is applied in form ofmechanical vibration, the penetrable material further needs to becapable to stand the vibratory friction of the anchoring step.

A penetrable material suitable for the method according to the inventionis solid at least under the conditions of the method according to theinvention (step of establishing the interference fit and anchoringstep), wherein “solid” in the context of the present disclosure is tomean that this material is rigid, substantially not elastically flexible(no elastomer characteristics) and not plastically deformable and it isnot or only very little elastically compressible. It further includes(actual or potential) spaces into which the liquefied material can flowor be pressed for the anchoring. It is e.g. fibrous or porous orincludes penetrable surface structures, which are, e.g., manufactured bysuitable machining or by coating (actual spaces for penetration).Alternatively the penetrable material is capable of developing suchspaces under the hydrostatic pressure of the liquefied thermoplasticmaterial, which means that it may not be penetrable or only to a verysmall degree when under ambient conditions. This property (havingpotential spaces for penetration) implies, e.g., inhomogeneity in termsof mechanical resistance. An example of a material that has thisproperty is a porous material whose pores are filled with a materialthat can be forced out of the pores, a composite of a soft material anda hard material or a heterogeneous material (such as wood) in which theinterfacial adhesion between the constituents is smaller than the forceexerted by the penetrating liquefied material. Thus, in general, thepenetrable material includes an inhomogeneity in terms of structure(“empty” spaces such as pores, cavities etc.) or in terms of materialcomposition (displaceable material or separable materials).

For the method according to the invention to produce a suitablyrepeatable joining quality the penetrable material needs to havemechanical properties which are predictable and homogeneous enough forgiving the interference fit a predicable strength. As this is hardly thecase for living bone the method is not suitable for medical purposes.

In embodiments with an opening, for example a blind opening, into whichthe insert portion is inserted by introducing the insert portion in theopening the anvil object is subject to a considerable mechanical loaddue to the interference force. Depending to the composition of thepenetrable material or possibly other materials of the anvil object,there may be a risk of cracks or other damages being caused by theintroduction of the insert portion into the opening prior to the step ofanchoring.

For example, in embodiments, the anvil object may include a board shapedportion (for example by being board shaped or by including a boardshaped constituent), with broad surfaces and a narrow side face betweenthe broad surfaces, with the opening extending from the narrow side faceinwards (i.e., the opening has a mouth on the narrow side surface).

Generally, if necessary, measures may be taken for preventing cracks orother damages (bulging, or portions flaking off for example) fromarising. Especially, measures for reducing the opposite surface areasthat are subject to the interference force may be taken:

-   -   The cross sections of the insert portion and of the opening are        adapted to each other by not having a same shape so that the        interference fit is limited to only parts of the circumference        of the cross sections.        -   If the distance between parts of subject to the interference            force is similar to or smaller than a flow distance of the            thermoplastic material during the subsequent anchoring step,            then nevertheless the anchoring may concern the full            circumference, with portions between the interference force            portions serve as additional space where the squeezed            thermoplastic material can go to. For example, if the cross            section of the opening is round and the cross section of the            insert portion is star shaped with closely spaced teeth,            then the thermoplastic material during the anchoring may            flow to fill spaces between the teeth.        -   If the distance between the interference force portions is            large, then anchoring will be accordingly limited to parts            of the circumference, essentially corresponding to the            interference force portions.        -   In embodiments of this concept, edges of such cross sections            may function as energy directing elements and/or may be            pressed into the lateral wall of the opening on establishing            the interference fit.    -   In addition or as an alternative, the cross section pairing of        insert portion and opening may differ along the depth of the        opening such that the interference fit is restricted to        predetermined areas arranged along this depth. Especially, the        interference fit may be restricted to areas at a distance from        the mouth of the opening (to deeper portions) because the risk        of cracks or other damages is the highest at the rim of the        opening.        -   In examples, the opening may be stepped or tapered with a            lower diameter at deeper positions, whereas the insert            portion does not have any steps/taper or is stepped/tapered            differently.    -   In addition or as yet another alternative, the insert portion        may be provided with a hollow part, especially if it is of a,        for example, slightly elastic material. In examples, the distal        end may be hollow.    -   In addition or as a further alternative, the insert portion may        be provided with stiff or flexible/soft wings. The insert        portion may then, for example, be shot into the opening by air        pressure or similar. The wings may ensure that the insert        portion is located in a correct position without causing a        cracking.    -   In addition or as an even further alternative, the interference        force may be caused only immediately prior to the anchoring step        and after the objects have been positioned relative to one        another, from an inside of the first object. Especially, the        first object may be provided with an interior opening accessible        from the proximal side, and an expansion element may be pushed        into the opening for example by the vibrating tool by which        thereafter the energy is transferred to the first object. The        expansion element in this has a larger cross section than the        interior opening so that it expands the first object and thereby        causes the interference force from inside.

Examples of penetrable materials applicable in the method according tothe invention are solid materials such as wood, plywood, chipboard,cardboard, concrete brick material, porous glass, foams of metal,ceramic, or polymer materials, or sintered ceramic, glass or metalmaterials, wherein such materials include spaces into which thethermoplastic material can penetrate which spaces are originally filledwith air or with another displaceable or compressible material. Furtherexamples are composite materials that have the above stated propertiesor materials with surfaces including a suitable roughness, suitablemachined surface structures or suitable surface coatings (e.g.consisting of particles). If the penetrable material has thermoplasticproperties it is necessary that it maintains its mechanical strengthduring the anchoring step either by further including a mechanicallystable phase or by having a considerably higher melting temperature thanthe thermoplastic material to be liquefied in the anchoring step.

A thermoplastic material suitable for the method according to theinvention is, under the conditions of the step of establishing theinterference fit, also solid in the sense as above described for thepenetrable material. It preferably includes a polymeric phase(especially C, P, S or Si chain based) that transforms from solid intoliquid or flowable above a critical temperature range, for example bymelting, and re-transforms into a solid material when again cooled belowthe critical temperature range, for example by crystallization, wherebythe viscosity of the solid phase is several orders of magnitude (atleast three orders of magnitude) higher than of the liquid phase. Thethermoplastic material will generally include a polymeric component thatis not cross-linked covalently or cross-linked in a manner that thecross-linking bonds open reversibly upon heating to or above a meltingtemperature range. The polymer material may further include a filler,e.g. fibers or particles of material that have no thermoplasticproperties or have thermoplastic properties including a meltingtemperature range that is considerably higher than the meltingtemperature range of the basic polymer.

Examples for the thermoplastic material applicable in the methodaccording to the invention are thermoplastic polymers, co-polymers orfilled polymers, wherein the basic polymer or co-polymer is, e.g.,polyethylene, polypropylene, polyamides (in particular Polyamide 12,Polyamide 11, Polyamide 6, or Polyamide 66), Polyoxymethylene,polycarbonateurethane, polycarbonates or polyester carbonates,acrylonitrile butadiene styrene (ABS), Acrylester-Styrol-Acrylnitril(ASA), Styrene-acrylonitrile, polyvinyl chloride, polystyrene, orPolyetherketone (PEEK), Polyetherimide (PEI), Polysulfon (PSU),Poly(p-phenylene sulfide) (PPS), Liquid crystall polymers (LCP) etc.LCPs are of particular interest since their sharp drop in viscosityduring melting enables them to penetrate in very fine spaces in thepenetrable material.

Usually either one of the two objects to be joined needs to be able totransfer the vibration energy, preferably with a minimum of energy loss,from a proximal object side, where a vibrating tool is applied, to adistal side, where the insert portion or the opening is arranged. Ifthis object is fully made of the thermoplastic material the latter willneed an elasticity coefficient (at ambient temperature) of at least0.5.GPa or preferably of at least 1.0 GPa.

The thermoplastic material and the penetrable material need to beadapted to each other such that a suitable interference fit and thedesired penetration are possible and result in a suitable anchorage. Amaterial pairing, which has proved to be advantageous is, e.g., thepairing of plywood (penetrable material) and polyamid (thermoplasticmaterial).

Mechanical vibration or oscillation suitable for the method according tothe invention has preferably a frequency between 2 and 200 kHz (evenmore preferably between 10 and 100 kHz, or between 20 and 40 kHz) and avibration energy of 0.2 to 20 W per square millimeter of active surface.The vibrating tool (e.g. sonotrode) is, e.g., designed such that itscontact face oscillates predominantly in the direction of the tool axis(longitudinal vibration) and with an amplitude of between 1 and 100 μm,preferably around 30 to 60 μm. Such preferred vibrations are, e.g.,produced by ultrasonic devices as known from ultrasonic welding.

In a preferred embodiment of the anchoring step of the method accordingto the invention one of the two objects (anvil object) is fixed in astable position and the vibrating tool is applied to the other object(free object) and, if applicable, the vibrating tool is not only usedfor transmitting the vibration to the free object but also a shearingforce being directed parallel to the depth of the opening. Therein thevibrating tool may not be connected to the free object or only loosely,i.e. it acts substantially as a hammer on the latter. Alternatively itmay be fixed to the free object such that the vibration is fullytransferred to the free object.

Undesired movement of the two objects relative to each other (or of theinsert portion within the opening respectively) due to the shearingforce directed parallel to the depth of the opening are limited orprevented by e.g. designing the opening as a blind opening or includinga relevant cross section reduction, or by arranging an auxiliary anvilwithin or at the mouth of a through opening and by positioning theinsert portion within the opening, in the step of achieving theinterference fit, such that its distal end abuts the bottom of the blindopening, or the cross section reduction, or the auxiliary anvil or has apredetermined small distance from the latter.

In an exemplary preferred embodiment of the method according to theinvention, the anvil object includes the opening (e.g., a blind bore,i.e., an opening with a constant circular cross section and a bottom)and it includes the penetrable material (e.g., it consists of wood orchipboard). The free object includes the insert portion (which is, e.g.,pin-shaped) and it consists of the thermoplastic material (e.g.polyamide, PA 6.6). Anchorage is to be achieved mainly on lateral sidesof the insert portion, but preferably also at the bottom of the bore.The insert portion has a cross section that is oversized relative to theopening by having a diameter being larger than the diameter of the bore.In the step of establishing the interference fit, the insert portion isforced into the bore until its distal end substantially contacts thebore bottom with the aid of the interference force, which is applied tothe proximal end of the pin using any suitable tool, possibly thevibrating tool in a passive state (not vibrating). In the anchoring stepthe vibrating tool is activated and, if applicable, is simultaneouslypressed against the proximal end of the free object with the additionalshearing force.

In embodiments of this kind, axial movement of the bulk of the insertportion may be restricted to compensation of compression of the bottomof the opening and penetration of the penetrable material in the area ofthis bottom. The proximal end of the insert portion may move further dueto shortening of the insert portion for compensating laterally displacedliquefied material.

The cylindrical insert portion may include energy directing features inform of axially extending ridges, which may or may not groove thelateral walls of the opening on establishing the interference fit. Ifthe oversize of the pin cross section is smaller than the radial heightof the ridges, the interference fit and therefore the anchorage will berestricted to the areas of the ridges and not concern the valleysbetween the ridges. Alternatively or in addition it is possible to equipthe lateral walls of the opening with energy directing structures. Theinsert portion is preferably fully made of the thermoplastic materialbut alternatively may include a core that extends substantially along acentral longitudinal axis of the insert portion and is made of amaterial (e.g., a metal or a ceramic material or a thermoplasticmaterial with a considerably higher melting temperature than thethermoplastic material to be liquefied) preferably having a highermechanical strength than the thermoplastic material.

Exemplary parameters for the above described preferred embodiment of themethod according to the invention, in which for the anchoring stepvibration as above described is used, are: (a) oversize of insertportion on lateral sides relative to lateral walls of opening: in theregion of tenths of millimeters (e.g. 0.1 to 0.5 mm), (b) interferencefit: in the region of tenths of N per mm² (e.g. 0.1 to 1 N/mm²).

Experiments show that using the above briefly described preferredembodiment of the method according to the invention the anchoring stepcan be reduced to about half of the time needed if, according to theknown method, the insert portion is forced into the openingsimultaneously with the application of the vibration. This reduction intime constitutes a relevant saving of vibratory energy and a relevantadvantage regarding an automated process. Furthermore, it puts lessstrain on a proximal face, on which the vibrating tool is hammering, andit reduces the risk of liquefied material being pressed out of the mouthof the opening. This increases the overall quality of the result of themethod in that, even if the named proximal face is made of thethermoplastic material, it is not deformed or undesirably marked duringthe anchoring step and the region of the mouth of the opening is keptclean.

Exemplary alternatives to the above named preferred embodiment of themethod according to the invention (as far as not already describedfurther above) are, for example:

-   -   Insert portion and opening are not cylindrical but e.g. tapering        either continuously or step-wise, wherein an overall tapering        angle is preferably not larger than 10 to 15°.    -   Any cross section of the insert portion is paired with an        opening in form of a groove, wherein a plurality of insert        portions may be introduced into the groove either side by side        or with distances therebetween.    -   The cross section of the insert portion and of the opening        differ by the one including energy directing elements and the        other one not.    -   The energy directing elements have the form of ridges extending        parallel to the depth of the opening.    -   At a proximal end of the insert portion, a proximal portion is        provided, wherein the proximal portion has, e.g., a larger cross        section than the insert portion such that the proximal portion        cannot be introduced into the opening. Such head-shaped proximal        portion may serve for application of the vibrating tool and        possibly for limiting movement of the insert portion into the        opening during the step of establishing the interference fit        and/or during the anchoring step. The proximal portion may in        addition have further functions, for example associated with its        shape.    -   If strong anchorage at the bottom of a blind opening is desired        this distal end of the insert portion is tapering or includes        energy directing surface structures.    -   If a minimum of anchorage at the bottom of a blind opening is        desired the distal end of the insert portion is preferably blunt        and/or does not include the thermoplastic material, and/or the        bottom of the opening includes a material that is not penetrable        or is less penetrable than the material of the lateral walls of        the opening.    -   The insert portion includes a core extending, e.g., in a        direction of the depth of the opening and consisting of a        material that does not soften or liquefy under the conditions of        the anchoring step (e.g. metallic or ceramic material or a        polymer material that is a thermoset polymer or a thermoplastic        polymer with a considerably higher melting temperature range        than the thermoplastic material that is to be liquefied in the        anchoring step). The core increases the mechanical strength of        the insert portion.

These possibilities may be combined with each other in arbitrarycombinations.

The invention also concerns a machine for carrying out the method in anautomated manner. Such a machine includes in insertion means forinserting the insert portion in the opening to establish theinterference fit, and an anchoring means for carrying out the anchoringprocess. The insertion means may be combined with the anchoring means(for example by a vibrating tool firstly pushing the insert portion intothe opening and then being subject to mechanical vibrations to coupleenergy into the arrangement for the anchoring process) or may beseparate therefrom.

The invention further concerns a set of a machine and a stock of firstobjects. If the machine further includes an opening manufacturing means(such as a drill), the opening manufacturing means and the first objectsare adapted to each other so that the interference fit can beestablished by insertion of the insert portion in the opening.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in further detail in connection with theappended Figs., wherein:

FIG. 1 illustrates the already above briefly discussed example of apreferred embodiment of the method according to the invention;

FIGS. 2A and 2 b show two examples of vibration transfer from thevibrating tool to the free object;

FIGS. 3A and 3B show two examples for the pairing of distal end of theinsert portion and bottom of the opening anchorage at the bottom of theopening in addition to the lateral anchorage;

FIG. 4 is a flow diagram of the method as illustrated in FIG. 1;

FIGS. 5 to 8 show examples of matched pairs of cross sections of insertportions and openings applicable e.g. in the method as illustrated inFIG. 1;

FIGS. 9 and 10 show examples of free objects with a not liquefiablecore;

FIGS. 11 to 16 show examples of longitudinal sections of matched insertportions and openings applicable, e.g., in the method as illustrated inFIG. 1;

FIGS. 17 and 18 illustrate examples of proximal portions connected toinsert portions anchored in an opening, e.g., in the method asillustrated in FIG. 1;

FIG. 19 illustrates an embodiment of an insert portion with a hollowdistal end;

FIG. 20 illustrates an embodiment of the method according to theinvention in which the interference fit is established by clamping wallparts of the opening around the insert portion;

FIG. 21 shows an example of an embodiment with an insert portion with aplurality of wings;

FIGS. 22 and 23 illustrate further examples of embodiments with a crosssection pairing of insert portion and opening differing along the depthof the opening such that the interference fit is restricted topredetermined areas arranged along this depth;

FIG. 24 shows an example of an embodiment in which the interferenceforce is caused from an inside of the insert portion; and

FIG. 25 schematically depicts a machine for carrying out a process asdescribed herein.

In the figures, same reference numbers refer to same or analogouselements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the above already briefly described preferredembodiment of the method according to the invention, wherein the twoobjects, or the insert portion and the opening respectively, are shownin section parallel to a depth of the opening (longitudinal section). Onthe left hand side of FIG. 1 the two objects to be joined areillustrated.

The anvil object 1 includes the blind opening 2 having a depth D, alateral wall 3, being substantially parallel to or forming an only smallangle with the depth D, and a bottom 4, being e.g. substantiallyperpendicular to the depth D. The anvil object 1 further includes thepenetrable material, which is arranged to constitute at least part ofthe lateral wall 3, e.g. the total of the lateral wall and in additionthe bottom wall. Therein the anvil object 1 may be made fully of thepenetrable material or may further include portions of other materials.The free object 5 includes a distally arranged insert portion 6 and itfurther includes the thermoplastic material, which constitutes at leastpart of the lateral surface 7 of the insert portion 6. Therein the freeobject 5 may be fully made of the thermoplastic material, as illustratedor only partly.

Insert portion 6 and opening 2 are adapted to each other in the abovedescribed manner (interfering cross sections, axial length of insertportion sufficient for enabling abutment of its distal end 10 on bottom4). The free object 5 may further include a proximal portion 8 (e.g.shaped like a head), wherein a proximal face 9 thereof is equipped forapplying tools used during the step of establishing the interference fitand during the anchoring step.

In the middle of FIG. 1 the two objects 1 and 5 are shown afterestablishment of the interference fit. The distal end 10 of theinsertion portion 6, which is, e.g., blunt, abuts the bottom 4 of theopening 2 and the insert portion 6 is retained in the opening 2 by aninterference fit, which acts mainly laterally (lateral surfaces ofinsert portion and lateral wall of opening are pressed against eachother). For positioning the insert portion 6 in the opening 2 theinterference force 20 is applied to the proximal face 9 of the freeobject 5 using any suitable tool (not illustrated). Alternatively, theinterference fit can be established by providing the anvil object 1,e.g., in a plurality of sections which all include parts of the openingand by closing the sections around the insert portion 6, wherein theforce by which the sections are moved against each other constitutes theinterference force (see FIG. 20).

On the right hand side of FIG. 1 the two objects 1 and 5 are shown whenjoined, i.e. after the anchoring step. The free element 5 is anchored inthe opening 2 in the area of its lateral surfaces 7 (or in the region ofthe lateral wall 3 of the opening 2 respectively) and in the region ofthe distal end 10 of the insert portion (or the bottom 4 of the opening2 respectively). The anchoring step is carried out with the help of avibrating tool 21, which is pressed against the proximal face 9 of thefree object 5.

The length L of the insert portion 6 being originally greater than thedepth D of the opening 2 allows movement of the free object 5 or itsproximal portion 8 respectively against the anvil object 1 forcompensating for the liquefied material, which is displaced during theanchoring step. At the latest when the proximal portion 8 abuts thesurface of the anvil object 1 the anchoring step is to be terminated,unless further anchoring is desired between the distal face of theproximal portion 8 and the surface of the anvil object 1. Termination ofthe anchoring step can alternatively or in addition be determined by apredetermined time interval for the anchoring step or by a predeterminedmaximum value of the shearing force 22.

FIGS. 2A and 2B illustrate two alternatives for carrying out theanchoring step, which alternatives regard the interface between thevibrating tool and the proximal face of the free object as brieflymentioned already further above.

For the alternative as illustrated in FIG. 2A the vibrating tool 21 isonly pressed against (not connected to) the proximal face of the freeobject 5 with the aid of the shearing force 22, wherein it is preferableto design the vibrating tool for maximal longitudinal vibration at itsdistal face. This means that only the one part of the vibration directedagainst the anvil object are transferred to the free object 5 (hammeringeffect, illustrated by a plurality of small arrows 23). If, with such anarrangement only lateral anchorage is to be achieved, care has to betaken, that there still is room at the bottom of the opening for a smalldisplacement of the distal end of the insert portion and the shearingforce 22 may be small. If anchorage also at the bottom of the opening isdesired as illustrated in FIG. 1, the shearing force does not only helpto overcome the interference fit for enabling the friction but it alsoserves the bottom anchorage as known from e.g. WO 98/042988.

For the alternative as illustrated in FIG. 2B, the vibrating tool 21 isrigidly connected to the free object 5, the connection being able totransmit the total vibration (double arrow 24) to the free object 5. Ifno anchorage at the bottom is desired, if the vibratory energy is largeenough for overcoming the interference fit, and if the anvil object issufficiently stably fixed or has a sufficient inertia, no additionalshearing force is necessary for achieving lateral anchorage. Such methodis in particular applicable when the opening is not a blind opening buta through tunnel. If the opening still includes a bottom and anchoragein this bottom in addition to the lateral anchorage is desired, asufficient “shearing” force for such bottom anchorage is needed, whichin addition may supplement the vibration for overcoming the interferencefit.

FIGS. 3A and 3B illustrate two alternatives for achieving strongerbottom anchorage, wherein only the distal end of the insert portion 6 ofthe free object 5 and the opening 2 of the anvil object 1 are shown.According to FIG. 3A a tapering distal end 10 of the insert portion 6 ispaired with a substantially flat bottom surface 4 of the opening 2.According to FIG. 3B a substantially blunt distal end 10 of the insertportion 6 is paired with an opening bottom 4 including a cone or similarshape having at least one elevation. While the alternative according toFIG. 3A favors interpenetration parallel to the depth of the opening,the alternative according to FIG. 3B favors lateral diversion of theliquefied material.

FIG. 4 is a flow diagram of the method according to the invention. Thisdiagram shows that the method includes the following steps: step 25 ofproviding the two objects, step 26 of establishing the interference fitbetween the insert portion and the opening with the aid of theinterference force, step 27 of anchoring the insert portion in theopening with the aid of mechanical vibratory energy and possibly theadditional shearing force, and finally step 28 of stopping the vibrationand of re-solidification of the material liquefied in the anchoringstep. The time sequence of steps 25 to 28 is the illustrated one andcannot be altered. However, there may be pauses between step 25 and 26and/or between steps 26 and 27, wherein, as mentioned further above, apause between step 25 and 26 has to be carefully planned if either oneof the thermoplastic and the penetrable material have creepingtendencies.

FIGS. 5 to 8 illustrate exemplary pairs of cross sections of insertportion and opening adapted to each other to be suitable at least forthe preferred embodiment of the method according to the invention asillustrated in FIG. 1. Therein the cross section 30 of the insertportion is shown with a continuous line and is projected over the crosssection 31 of the opening, which is shown with a broken line. The drawncross sections 30 and 31 are the cross sections of the provided elements(before the steps of establishing the interference fit and before theanchoring step). The oversize of the insert portion is not drawn inscale (too large). For strictly cylindrical (circular or other cylinder)insert portions and openings the shown cross sections 30 and 31 willextend over most of the axial insert length and opening depth. For otherforms of insert portions and/or openings the cross sections may changein form and/or size along the axial insert length and/or opening depth.

FIG. 5 shows two circular cross sections 30 and 31 with the requiredoversize of the cross section 30 of the insert portion. Such a crosssection pairing will result in a uniform anchorage on the wholecircumference of insert portion and opening provided that the penetrablematerial of the lateral opening wall is uniform in the areas of theinterference fit.

FIG. 6 shows a circular cross section 31 for the opening and a crosssection 30 for the insert portion with a plurality of ribs 32. The ribs32 serve as energy directing structures. If sufficiently sharp, the ribs32 may penetrate the lateral wall of the opening on establishing theinterference fit, which has to be taken into consideration whendetermining the interference of the cross sections. If only the ribs areoversized but not the valleys therebetween, anchorage will only occur inthe vicinity of these ribs. Similar effects can be achieved by equippingthe cross section 31 of the opening with points protruding from theopening wall while still keeping the cross section 30 of the insertportion substantially circular.

FIG. 7 shows a pairing of a circular cross section 31 for the openingand a cross section 30 for the insert portion, which has indentations 33of a sufficient depth, such that in their area no interference fit willbe established and therewith no anchorage will occur. Also in this casethe indentations may be provided in the wall of the opening instead ofin the insert portion.

In the embodiment if FIG. 8, the insert portion in at least one depthhas a star shaped cross section. The star-shaped cross section includessharp edges 32 that like in the embodiments of FIGS. 6 and 7 may serveas energy directors. The depth of the indentations 33 between the edgesis such that even if the edges cut into the penetrable material, thereremains some space into which in the anchoring step some thermoplasticmaterial may flow. This may especially be advantageous in case thepenetrable material has a comparably low porosity.

In contrast to the embodiments of FIGS. 6 and 7, in the embodiment ofFIG. 8 anchorage may occur along the full periphery of the insertportion.

FIG. 9 depicts a free object 5 with a core 82 of a for example metallicmaterial embedded in a thermoplastic coating 71.

In embodiments with the object that includes the thermoplastic materialfurther includes material that does not soften or liquefy under theconditions of the anchoring step, the thermoplastic material does notneed to constitute the entire surface. Rather, it is sufficient if atleast part of the surface area, loaded by the interference fit isconstituted by the thermoplastic material. FIG. 10 shows an examplewhere the thermoplastic material 81 is only present at the distal end.Alternatives with for example axially extending strips of thethermoplastic material are possible, too.

FIGS. 11 to 16 show pairings of insert portions and openings suitablefor the method according to the invention, wherein the insert portionsand openings are sectioned along the insert portion length or theopening depth respectively, and wherein again the section 35 of theinsert portion is shown with a continuous line and is projected over thesection 36 of the opening, which is drawn as broken line. Again thesections 35 and 36 are the sections of the elements as provided (i.e.before the step of achieving the interference fit and before theanchoring step).

FIG. 11 shows a pairing of a tapering insert portion with a taperingopening, wherein the tapering is consists of a plurality of steps 37,wherein the steps on insert portion and opening have substantially thesame axial lengths, and wherein the cross sections of insert portion andopening between steps are adapted to each other in substantially thesame manner (similar oversize).

FIG. 12 shows a pairing of a tapering insert portion with a taperingopening, wherein the tapering in both cases is continuous and has anangle A relative to the insert portion length and the opening depth ofnot more than 10 to 15°. The cross sections of insert portion andopening over insert portion length and opening depth have preferably allsimilar forms but decreasing sizes.

FIG. 13 illustrates the already further above discussed embodiment usingan auxiliary anvil 40 positioned in a through opening and serving forcounteracting the shearing force and possibly also the interferenceforce. If the auxiliary anvil 40 is to be removed from the opening, careis to be taken when designing the anvil surface and the distal end ofthe insert portion that no connection between the two is effected in theanchoring step. Like the bore bottom of FIG. 3B, the auxiliary anvil mayhave a face directed towards the insert portion including an elevation,which may function as energy directing element for liquefaction at thedistal end of the insert portion.

FIG. 14 illustrates the already further above discussed embodiment inwhich movement of the insert portion in the opening is limited by a step41 in the opening where the cross section of the opening is reduced insuch a manner that the distal end of the insert portion cannot be movedpast the step neither under the influence of the interference force norunder the influence of the shearing force.

FIG. 15 illustrates a further exemplified embodiment of the methodaccording to the invention wherein the object 5 including the insertportion and the thermoplastic material constitutes a sort of blindrivet. A distal portion 5.1. of the object 5 is to be anchored in anopening of a first object 1.1 including the penetrable material and aproximal portion 5.2 of the object 5 is to be anchored in an opening ofa second object 1.2 including the penetrable material. Therein theinterference force, the vibration and, if applicable, the shearing forceare preferably applied to object 1.2 (free object) and object 1.1 iskept stable (anvil object) wherein object 5 functions as both free andanvil object.

FIG. 16 illustrates a pairing of longitudinal sections 35 and 36 ofinsert portion and opening in which interference fit and anchorage arelimited to predefined areas alternating with areas of no interferencefit and anchorage along the longitudinal axis of the insertion portionor the opening depth respectively. This is achieved by cross sections ofalternating sizes. The same effect can be achieved by pairing acylindrical insert portion with an opening including portions ofdiffering cross section sizes or vice versa. If the vibrating tool 21 isdesigned and energized for longitudinal vibration it is advantageous toarrange the above named areas of interference fit and anchorage atlocations with large amplitude and the above named areas with nointerference fit and no anchorage at locations with small amplitude.This means with maximum longitudinal amplitude at the distal face of thevibrating tool and the proximal face of the free object, that areas withinterference fit and anchorage are preferably arranged at distances fromthe proximal face of n times λ/2 (n=integer) and areas of nointerference fit and no anchorage therebetween (at distances of λ/4,3λ/4, 5λ/4 etc.). This is shown in the diagram on the right hand side ofFIG. 16.

FIGS. 17 and 18 show further exemplary embodiments of proximal ends ofinsert portions anchored in openings, which are achievable with themethod according to the invention and in particular with the preferredembodiment thereof as illustrated in FIG. 1. Both FIGS. 17 and 18 arepartial sections parallel to the axial length of the insert portion orthe opening respectively, wherein only a mouth region of the opening anda proximal portion of object 5 are shown.

FIG. 17 shows an object 5 with an insert portion only (no proximalportion), which is anchored in the opening with its proximal face 9flush with the surface of the object 1 or in the mouth of the opening,respectively. This is achieved by dimensioning the object 5 with alength greater than the opening depth wherein the differencesubstantially corresponds with the expected shortening of object 5during the anchoring step due to thermoplastic material penetrating thelateral and bottom wall of the opening. In the same manner, i.e. bycorresponding difference between insert portion length and opening depthit is possible to achieve any desired position of the proximal face 9relative to the mouth of the opening.

FIG. 18 shows a headed object 5, e.g. similar to the one shown in FIG.1, wherein the head-shaped proximal portion 8 is countersunk in acorresponding enlargement of the opening cross section adjacent to themouth of the opening.

FIG. 19 illustrates an embodiment of an insert portion 6 with a hollowdistal end. The distal end of the insert portion 6 is provided with ahollow space 61 so that the distal end 10 is somewhat flexible andprovides less resistance to deformations when the insert portion issubject to a radial compression force, compared to an embodiment with ablunt distal end. Due to this, during introduction into the opening theinterference force rises gradually instead of being particularly stronginitially.

FIG. 20 illustrates an embodiment of the method according to theinvention in which, as mentioned already further above, the interferencefit is established not by forcing the insert portion into the openingbut by positioning the insert portion between wall parts of an openingand clamping the wall parts against the insert portion. In theillustrated exemplary embodiment the wall parts are constituted bynarrow surfaces of boards (anvil objects 1.3 and 1.4), e.g. chipboards,which extend substantially parallel and which constitute between themthe opening 2. The insert portion 6 has the form of a lath. In the stepof establishing the interference fit, the insert portion is positionedand clamped between the wall parts wherein the anvil objects 1.3 and 1.4are pressed against the free object 5 (interference force 20). FIG. 20shows above the objects before the step of establishing the interferencefit and below after the anchoring step.

In the embodiment of FIG. 21, the free object 5 has a plurality of wings62 attached to the insert portion 6. The wings 62 may be comparably softand thereby assist the location of the insert portion 6 in the correctposition in the opening 2 without causing cracking in the anvil object1. Especially, in embodiments, the cross section of the insert portionbody (i.e. that part of the insert portion that is not constituted bythe wings 62) may approximately correspond to the cross section of theopening 2 or be even slightly lower than the latter so that theinterference force is only present at the location of the wings 62. Thisembodiment may be suited for especially weak or brittle penetrablematerial.

In a variant, the wings 62 may be dimensionally stable and serve ascutting blades. Also in this variant, the wings 62 may stabilize theinsert portion during insertion.

Embodiments with the wings 62 are examples of embodiments that areparticularly suited for processes in which the insert portion 6 is shotinto the opening, for example comparable to an air gun like process.

FIG. 22 shows, further to FIG. 16, an example of an embodiment with across section pairing of insert portion and opening differing along thedepth of the opening such that the interference fit is restricted topredetermined areas arranged along this depth. Especially, in theembodiment of FIG. 22 the interference fit is restricted to deepersections thus minimizing the risk of cracks or other damages at the rimof the opening where the second object 1 is weakest, especially if it isboard shaped and the opening extends from the narrow face.

To this end, the opening is stepped, whereas the insert portion isessentially cylindrical (with the possible exception of energy directorsand/or a slight taper).

An opening having an inner (distal), smaller diameter section 2.1 and anouter (proximal) larger diameter section 2.2 may for example bemanufactured by a dual drilling process, with two different drills. Alsodrills with a step feature are possible.

The cross section (diameter d₁ in the depicted embodiment assuming anapproximately circular cross section) of the inner section 2.1 will besmaller than the cross section (diameter d_(i)) of the insert portion togenerate the interference fit, whereas the cross section (diameter d₂)of the outer section 2.2 may approximately correspond to the diameter ofthe insert portion or may even be slightly larger.

FIG. 23 illustrates a variant of the embodiment of FIG. 22 with multiplesteps. The diameters of the respective sections are such that nointerference fit is present in the depicted configuration where theinsert portion is only partially inserted, and an interference fit willarise if the distal-most section 6.2 of the insert portion is pressedinto the innermost portion 2.1 of the opening and the more proximalsection 6.1 enters the middle section 2.2 of the opening but not withinthe proximal-most section 2.3 of the opening.

FIG. 24 shows a further embodiment. In this, the anvil object 1 isassumed to be a board having two broad surfaces 81, 82 a narrow sideface 83, with the opening 2 in the narrow side face. In embodiments witha board shaped section of the anvil object, there is a particular riskof cracks generated upon introduction of the insert portion in theopening, or of portions flaking off).

The embodiment of FIG. 24 is an example of an embodiment in which theinterference force is caused from an inside of the free object 5,starting, for example, only immediately prior to the anchoring step. Tothis end, the free object 5 is provided with an interior opening 70 intowhich an expansion element 71 is pushed, the expansion element having atleast in some axial depths a cross section larger than a cross sectionof the interior opening. The insert portion 6 and the opening 2 may beadapted to each other so that no interference fit or an interference fitwith only a small interference force is established when the insertportion 6 is in the opening 5 before the expansion element 71 is placed.The expansion element 71 may for example be introduced by a forceexerted by the vibrating tool 21 (sonotrode) immediately prior to thestep of causing the mechanical vibration to impinge on the free objectby the vibrating tool 21.

A configuration with the anvil object having a board shaped section andwith the opening extending from the narrow side face is applicable toall embodiments described in this text. Applications include thefurniture industry, where a head portion 8 of the free object 5 mayserve male part of a joint co-operating a female parts of a secondfurniture part, for example as described in as WO 2013/104422 by InterIkea Systems B.V.

FIG. 25 yet schematically illustrates elements of a machine for carryingout the method described herein in an automated manner. The machineincludes, in addition to the vibrating tool 21, a gripping arrangement91 for positioning the free object 5 relative to the anvil object 1prior to the step of establishing the interference fit and also, atleast in part, during this step. For establishing the interference fit,the vibrating tool 21, prior to being subject to the vibrations, may beused, for example by pressing the insert portion into the opening.Alternatively, the machine may include a separate insertion station,where the interference fit is established by a corresponding insertiontool, for example a pusher tool.

The vibrating tool is connected to a vibration source 92, such as apiezoelectric transducer, that causes the vibrating tool to vibrate.

The machine may optionally further include an opening producing device,such as a drilling device 93, including a drill 94 the diameter of whichis adapted to an outer diameter of the insert portion 6 in a manner thatwhen the insert portion is inserted in the opening the interference fitis established, i.e. the drill 94 is configured to form an opening witha diameter somewhat less than an outer diameter of the insert portion.

The machine may be an automated machine for mass production, includingseveral stations and transport means for conveying the anvil object fromstation to station. For example, the machine may include an openingforming (drillings) station, an insertion station, and an anchoringstation, as well as means for conveying anvil objects from station tostation in a cycled manner. A machine may include further stations, suchas a cutting-to-size station, arranged before the opening formingstation. In addition or as an alternative, it may include storages forunprocessed anvil objects, processed anvil objects (to which the objectsare conveyed after the method has been carried out) and/or for freeobjects.

Example

An anchoring device of polyamide (PA 6.6 filled with 15% of glassfibers) was joined to a porous structural foam of a density in the rangeof 0.5 to 0.8 g/cm³, using the method as illustrated in FIG. 1. Theopening provided in the foam was a blind bore. The inserted portion ofthe device was cylindrical, had a circular cross section (no energydirecting structures) with a diameter being 3% oversized to create theinterference fit, an overlength of 25% with respect to the bore depth,and a blunt distal end. The device was forced into the opening whichnecessitated an interference force of about 10 MPa related to the devicecross section. In the inserted position the distal end of the deviceabutted the bottom of the blind bore. For the anchoring step, the foamwas locally fixed (anvil object) and ultrasonic vibration was coupledinto the proximal face of the device (free object) and simultaneouslythe pin was pressed against the bottom of the opening with a shearingforce of about twice the interference force, using a stationaryultrasonic device by Branson (Series 2000). Therein the device was notattached to the sonotrode of the ultrasonic device. The anchoring stepwas carried out with a vibration frequency of about 20 kHz and alongitudinal amplitude at the distal end of the sonotrode in the rangeof 50 to 90 μm. After about 0.3 sec of anchoring time the device wasfully seated in the foam and the vibration was stopped.

The overlength of the device was consumed by displacement of thethermoplastic material into the bottom of the opening, and possibly alsoby an increase of the depth of the opening due to either theinterference force or the shearing force, and further by radialdisplacement of the thermoplastic material. The anchorage on the lateralsides of the device was very uniform all around the device and inparticular along its length. For removing the device from the foam aforce had to be applied which was about twice the force needed forremoval of an identical device anchored with a similar method in anon-undersized bore (no lateral interference fit and therefore hardlyany lateral anchorage). This allows the conclusion that the lateralanchorage induced by the lateral interference fit contributes about thesame amount of retention strength as the anchorage at the bottom of thebore.

If a similar device is anchored in a similar opening using the sameequipment but applying the vibration already during the forcing of thepin into the opening, the bulk of the device needs to be moved duringthe anchoring step for the full opening depth, which at least doublesthe time needed for the anchoring step.

1. A method of joining two objects, the method comprising the steps of:providing the two objects one of the two objects comprising a firstmaterial and the other one of the two objects comprising a secondmaterial, wherein the first material is solid and comprisesthermoplastic properties and wherein the second material is solid and ispenetrable by the first material when in a liquefied state, one of thetwo objects further comprising an opening having a depth and the otherone of the two objects further comprising an insert portion having alength, wherein the opening and the insert portion are adapted to eachother for the insert portion to be positioned in the opening with aninterference fit, and wherein said first and second materials constituteat least part of opposite surface areas of insert portion and openingpressed against each other in the interference fit, establishing theinterference fit by placing the insert portion in the opening andapplying an interference force; after establishing the interference fit,anchoring the insert portion in the opening by transferring energysuitable for liquefaction of the first material to the vicinity of saidopposite surface areas in an amount and for a time sufficient, forliquefaction of the first material and interpenetration of the first andsecond materials in the vicinity of said opposite surface areas;stopping the transfer of energy for a time sufficient for the firstmaterial liquefied during the step of anchoring to re-solidify.
 2. Themethod according to claim 1, wherein, in the step of anchoring, thetransferred energy is mechanical vibration energy.
 3. The methodaccording to claim 2, wherein, in addition to the mechanical vibrationenergy, a shearing force is applied to the interference fit, theshearing force acting between the two objects.
 4. The method accordingto claim 1, wherein, the insert portion is oriented within the openingwith the length substantially parallel to the depth, and wherein theinterference fit is established between lateral sides of the insertportion and a lateral wall of the opening.
 5. The method according toclaim 4, wherein the interference fit is established by forcing theinsert portion into the opening with the aid of the interference forcebeing directed substantially parallel to the length and to the depth andbeing applied to a proximal face of one of the two objects.
 6. Themethod according to claim 3, wherein the shearing force is directedsubstantially parallel to the length and to the depth and is applied toa proximal face of one of the two objects.
 7. The method according toclaim 6, wherein, during the step of anchoring, movement of the twoobjects relative to each other caused by the shearing force is preventedor limited.
 8. The method according to claim 7, wherein said movement isprevented or limited by a distal end of the insert portion beingpositioned to abut a bottom of the opening, by a restriction in anopening cross section or by an auxiliary anvil positioned in theopening.
 9. The method according to claim 1, wherein the one of the twoobjects that comprises the opening also comprises the second material,and wherein the other one of the two objects that comprises the insertportion also comprises the first material.
 10. The method according toclaim 2, wherein the interference force, the mechanical vibration energyand, if applicable, the shearing force are applied to the one of the twoobjects that comprises the insert portion and the first material. 11.The method according to claim 9, wherein the one of the two objectscomprising the first material is completely formed from the firstmaterial.
 12. The method according to claim 9, wherein the one of thetwo objects comprising the first material further comprises a core of afurther material which under the conditions of the anchoring step is notsoftened or liquefied.
 13. The method according to claim 9, wherein thelateral sides of the insert portion comprise energy directing structuresin the form of ridges extending substantially parallel to the length.14. The method according to claim 4, wherein the opening and the insertportion are substantially cylindrical or conical.
 15. The methodaccording to claim 1, wherein the second material is solid during thesteps of establishing the interference fit and of anchoring.
 16. Themethod according to claim 15, wherein the second material is one offibrous, porous, comprising penetrable surface structures.
 17. Themethod according to claim 15, wherein the second material is one ofchipboard, fibre board, wood, plywood and cardboard.
 18. The methodaccording to claim 1, wherein the first material comprises athermoplastic polymer with an elasticity coefficient of at least 0.5GPa.
 19. The method according to claim 18, wherein the first materialfurther comprises a filler.
 20. The method according to claim 1, whereincross sections of the insert portion and of the opening are adapted toeach other by not having a same shape so that the interference fit islimited to only parts of the circumference of the cross sections. 21.The method according to claim 1, wherein cross section pairing of theinsert portion and the opening differs along the depth of the openingsuch that the interference fit is restricted to predetermined areasalong the depth.
 22. The method according to claim 21, wherein theinterference fit is restricted to areas that do not extend to a mouth ofthe opening, wherein the mouth of the opening preferably issubstantially free from interference fit.
 23. The method according toclaim 1, wherein the insert portion comprises an interior openingaccessible from a proximal side of the one of the object that comprisesthe insert portion, wherein the step of establishing the interferencefit comprises the sub-steps of placing the insert portion relative tothe opening and of thereafter inserting an expansion element in theinterior opening to cause an expansion of the insert portion within theopening for establishing the interference fit.
 24. A machine forcarrying out the process according to claim 1, comprising in insertionmeans for inserting the insert portion in the opening to establish theinterference fit, and an anchoring means for carrying out the step ofanchoring.
 25. The machine according to claim 24, comprising aninsertion station with the insertion means, and an anchoring stationseparate therefrom,
 26. The machine according to claim 25, furthercomprising a conveyor for conveying second objects from the insertionstation to the anchoring station.
 27. The machine according claim 24,further comprising an opening manufacturing station with an openingmanufacturing means for manufacturing the opening prior to the step ofinserting.
 28. A set comprising a machine according to claim 27 andfurther comprising a stock of first objects, the insert portions of thefirst objects and the means for manufacturing the opening being adaptedto each other so that the interference fit can be established uponinsertion of the insert portion into the opening.